1. Field of the Invention
This invention relates to an improved method of preparing esters of rosin. In particular, this invention relates to an improved rosin-polyol ester prepared by reacting rosin and a polyhydric alcohol in the presence of phosphinic acid (H.sub.3 PO.sub.2) and a phenol sulfide compound, as combined catalyst, wherein the acidity in the product ester is neutralized with a magnesium salt. Most particularly, the invention relates to employing the rosin ester as a tackifier in ethylene-vinyl acetate based hot melt adhesives to produce hot melts of improved clarity and viscosity stability.
2. Description of the Prior Art
Rosin is mainly a mixture of C.sub.20, fused-ring, monocarboxylic acids, typified by levopimaric and abietic acids, both of which are susceptible to numerous chemical transformations. The rosins to which this invention relates include gum rosin, wood rosin, and tall oil rosin or the rosin acids contained therein as for example, abietic acid, pimaric acid, sapinic acid, etc.
The natural separation and gradual conversion of some of the hydrophilic components of sap and related plant fluids from the cambium layer of a tree into increasingly hydrophobic solids are the generic process of forming diverse gums, resins and waxes. The oleoresin intermediate in this process is typified in pine gum, which flows from hacks on the trunks of southern yellow pine in southeastern United States, in France, and in other countries. Pine gum contains about 80% (gum) rosin and about 20% turpentine.
Resinification from oleoresin can result from either natural evaporation of oil from an extrudate or slow collection in ducts in sapwood and heartwood. Pinus stumps are valuable enough to be harvested, chipped, and extracted with hexane or higher-boiling paraffins to yield wood rosin, wood turpentine, and other terpene-related compounds by fractional distillation. In the kraft, i.e., sulfate, pulping process for making paper, pinewood is digested with alkali producing crude tall oil and crude sulfate turpentine as by-products. Fractionation of the crude tall oil yields tall oil rosin and fatty acids.
The chemical transformation of gum, wood, and tall oil rosin which relates to this invention is esterification. The beneficial product characteristics provided by rosin esterification for various applications have led to the development of many esterification procedures, particularly treatments with polyhydric alcohols. U.S. Pat. Nos. 2,369,125, 2,590,910 and 2,572,086 teach rosin esterification with glycerol and pentaerythritol, among other polyhydric alcohols, usually preceded by a rosin disproportionation step.
Commonly assigned U.S. Pat. No. 4,548,746 teaches the use of phosphinic acid (also called hypophosphorous acid) as a catalyst in the esterification of rosin with pentaerythritol to produce a rosin ester without appreciable color degradation. Also commonly assigned U.S. Pat. No. 4,650,607 discloses a method for rosin esterification in the presence of phosphinic acid and a phenol sulfide compound, as combined catalyst, to provide an ester of improved color and oxidation stability.
Hot melt adhesives based on ethylene-vinyl acetate (EVA) may contain 25-65% rosin-polyol ester as the tackifier. The compatibility of ester with the other hot melt components depends on the polyol, catalysts, additives and process used in making the ester. Good compatibility is indicated if the hot melt formulates clear and remains free of haze during thermal aging. The viscosity stability of the hot melt also is affected by the components and process used to make the ester. What gives good hot melt compatibility may give poor hot melt viscosity stability and vice versa.
The commercial EVA-based hot melt adhesive formulation which follows (designated as Formula A) is of relatively low polarity and is sensitive to variations in rosin ester tackifier compatibility.
______________________________________ Formula A ______________________________________ 35-45% Tall oil rosin-pentaerythritol ester 25-40% EVA copolymer (with a vinyl acetate content of 17.5-18.5% and a melt index of 136-165) 10-25% Synthetic wax (with a congealing point of 204-208.degree. F.) ______________________________________
The commercial EVA-based hot melt adhesive formulation which follows (designated as Formula B) is of relatively high polarity and is sensitive to variations in viscosity stability.
______________________________________ Formula B ______________________________________ 45-60% Tall oil rosin-pentaerythritol ester 20-40% EVA copolymer (with a vinyl acetate content of 27-29% and a melt index of 22-28) 10-25% Microcrystalline wax (with a melting point of 182.degree. F.) .about..5% Antioxidant ______________________________________
In the case of the low-color pentaerythritol ester wherein phosphinic acid together with a phenol sulfide compound catalyzes, bleaches, and stabilizes the product, the ester has excellent compatibility in a wide range of EVA-based hot melts if the acidity in the product is not neutralized, but the viscosity stability is very poor due to acid-catalyzed degradation of the EVA. Acceptable viscosity stability is achieved by neutralizing the ester with inorganic bases such as the alkali metal hydroxides so that the pH value of the ester, as measured in a 2:3 (volume:volume) mixture of methanol and toluene, is between about 6 and about 8. For example, U.S. Pat. No. 4,548,746 teaches neutralization with sodium hydroxide. However, the neutralized esters produce hazy hot melts either upon formulation or after thermal aging. While the cause of this hazing is not understood entirely, it is suspected that it may be due to the separation of inorganic salts of catalyst residues. Some improvement is obtained by neutralizing with calcium acetate, but haze still is observed even in more polar EVA hot melts with higher levels of vinyl acetate and is worse in non-polar hot melts. Organic amines can be used to neutralize the acidity, but they still give poor viscosity stability and may cause discoloration.
U.S. Pat. Nos. 4,168,253 and 4,284,543 teach the use of a lithium salt as an additive to the hydrocarbon tackified EVA-based hot melt adhesive compositions to extend the molten stability. However, as noted in the examples to follow, lithium hydroxide provides no improvement when employed as the ester neutralizing agent in the present invention.
The object of this invention, therefore, is to provide novel esters of rosin for use in hot melt adhesives, particularly ethylene-vinyl acetate based hot melts. It is a further object of this invention to employ an acidity neutralizing agent for certain rosin esters which makes the ester compatible as a tackifier for hot melt adhesives, reducing the degree of hazing during formulation or during thermal aging of the hot melt. It is a still further object of this invention to provide a compatible tackifier for hot melt adhesives which gives good viscosity stability to the hot melt. It is a specific object of this invention to provide a novel method of preparing esters of rosin exhibiting excellent compatibility and viscosity stability when employed as a tackifier for hot melt adhesives, particularly for ethylene-vinyl acetate based hot melts.